Method and apparatus for cooling down a cryogenic heat exchanger and method of liquefying a hydrocarbon stream

ABSTRACT

The present invention relates to a method and apparatus for cooling down a cryogenic heat exchanger adapted to liquefy a hydrocarbon stream, such as a natural gas stream. The method comprises:(i) receiving one or more refrigerant temperature indications, providing an indication of the temperature of the refrigerant,(ii) comparing the one or more refrigerant temperature indications with one or more associated predetermined threshold values, and(iii) based on the outcome of the comparison under (ii) selecting one of an automated warm cooling down procedure of the cryogenic heat exchanger and an automated cold cooling down procedure of the cryogenic heat exchanger.

CROSS REFERENCE TO EARLIER APPLICATION

The present application is a U.S. Divisional application of U.S.application Ser. No. 15/538,940, filed on Jun. 22, 2017, which is aNational Stage (§ 371) application of PCT/EP2015/081233, filed Dec. 24,2015, which claims priority benefits of European Application No.14200463.9, filed Dec. 29, 2014, the disclosure of which is incorporatedby reference herein.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for cooling downa cryogenic heat exchanger.

In various embodiments specifically disclosed herein, the cryogenic heatexchanger is adapted to liquefy a hydrocarbon stream, such as a naturalgas stream.

In another aspect, the present invention relates to a method ofliquefying such a hydrocarbon stream.

BACKGROUND

Several types of cryogenic heat exchangers are known. Such cryogenicheat exchangers may be used in methods of liquefying a natural gasstream to produce liquefied natural gas (LNG). In such a case, thecryogenic heat exchanger is generally able to receive the hydrocarbonstream to be liquefied, to heat exchange the hydrocarbon stream againstan at least partly evaporating refrigerant thereby at least partiallyliquefying the hydrocarbon stream, and to discharge the at leastpartially liquefied hydrocarbon stream.

Depending on the type of hydrocarbons in the stream, and the pressurelevel under which the hydrocarbon stream passes through the cryogenicheat exchanger, a typical temperature at which for instance natural gasstarts to liquefy may be at −135° C.

However, before it is ready for normal operation of cooling and/orliquefying the hydrocarbon stream, the cryogenic heat exchanger needs tobe cooled down, e.g. as part of a plant start-up routine.

In order to prevent damage to the cryogenic heat exchanger, includingfor instance leaks that may result from thermal expansion andcontraction distributions over the cryogenic heat exchanger, operatorsand manufacturers of such cryogenic heat exchangers typically recommendto avoid as much as possible to exceed a certain specified maximumtemperature rate of change over time.

On the other hand, in order to minimize the non-productive orsub-optimal productive period of the cryogenic heat exchanger, operatorstypically want to cool down their cryogenic heat exchanger at thehighest rate possible.

U.S. Pat. No. 4,809,154 describes an automated control system for thecontrol of mixed refrigerant-type liquefied natural gas productionfacilities, wherein functional parameters are optimized. Optimization isaccomplished by adjusting parameters including mixed refrigerantinventory, composition, compression ratio, and compressor turbine speedsto achieve the highest product output value for each unit of energyconsumed by the facility.

In more detail, process controller system of U.S. Pat. No. '154 isimplemented in a parallel processing computer system allowing parallelcontrol processes to be executed on multiple processors having access toa common storage wherein values representative of the current state ofevery sensor and every controller associated with the productionfacility are stored. To manage the parallel control processes, a requestqueue and a return queue are maintained, as well as a priority table,which is used to resolve contention among parallel operating processloops.

The process controller system of U.S. Pat. No. '154 may worksatisfactorily to optimize or keep optimal quantity or quality of theliquefied gas being produced while the liquefaction process runs.However, the process controller system of U.S. Pat. No. '154 is notsuitable for controlling the cryogenic heat exchanger during initialcooling down at start up, because that requires a sequence of steps tobe carried out which cannot be handled using the system of prioritytables and request and return queues.

WO2009/098278 describes a method and apparatus for cooling down acryogenic heat exchanger. Cooling down is done in an automated mannerand allows to cool down at the highest rate possible without exceedingthe specified maximum rate of temperature change.

SHORT DESCRIPTION

It is an object of the invention to provide an apparatus and method forcooling down a cryogenic heat exchanger in a more flexible and timeefficient manner, in particular in situations wherein operation of thecryogenic heat exchanger is restarted after an interruption, while therefrigerant is still well below ambient temperature. This may forinstance be the case if operation has been interrupted for a relativelyshort period of time (for instance for maintenance, after a compressorinduced trip, pit stop or a shutdown normally required) or even afterlonger interruptions (days) during which proper box-up is done tomaintain the low temperature as much as possible.

The present invention provides an apparatus for cooling down a cryogenicheat exchanger adapted to liquefy a hydrocarbon stream, such as anatural gas stream, which cryogenic heat exchanger is arranged toreceive the hydrocarbon stream to be liquefied and a refrigerant, toexchange heat between the hydrocarbon stream and the refrigerant,thereby at least partially liquefying the hydrocarbon stream, and todischarge the at least partially liquefied hydrocarbon stream and spentrefrigerant that has passed through the cryogenic heat exchanger, theapparatus comprising

-   -   a refrigerant recirculation circuit to recirculate spent        refrigerant back to the cryogenic heat exchanger, the        refrigerant recirculation circuit comprising at least a        compressor, a compressor recycle valve, a cooler, and a first JT        valve;    -   a programmable controller arranged to perform a comparison step        (502), comprising:        -   (i) receive one or more refrigerant temperature indications,            providing an indication of the temperature of the            refrigerant,        -   (ii) compare the one or more refrigerant temperature            indications with one or more associated predetermined            threshold values, and        -   (iii) based on the outcome of the comparison under (ii)            select one of an automated warm cooling down procedure of            the cryogenic heat exchanger and an automated cold cooling            down procedure of the cryogenic heat exchanger.

After step (iii) the programmable controller is arranged to execute theselected cooling down procedure. It is noted that the warm and the coldcooling down procedure are similar procedures which may involve similaractions, but the actions are executed in a different way, in particulardiffering in one of the following aspects: different step sizes foropening/closing valves, different timing for opening/closing valves,different threshold values for deciding on further opening/closingvalves, in particular different TROC-values.

In particular, the cold cooling down procedure differs from the warmcooling down procedure as the warm cooling down procedure comprises astep in which it is ensured that the JT valve is closed (closedautomatically or by prompting an operator to close the JT valve), whilethe cold cooling down procedure allows to start with an opened JT valve.The cold cooling down procedure allows to start reducing the cryogenicheat exchanger temperature from the (still) cold condition (e.g. −80° C.to −130° C.) down to LNG production point (approximately −165° C.). Thiswill be explained in more detail below.

The term warm cooling down procedure is used to refer to a cool downprocedure where the initial temperature of the refrigerant is relativelyhigh, e.g. above the predetermined threshold value, possibly requiring apre-cool down procedure. An example of a warm cooling down procedure isexplained in detail in WO2009/098278 which is hereby incorporated byreference.

The term cold cooling down procedure is used to refer to a cool downprocedure where the initial temperature of the refrigerant is relativelylow, e.g. below the predetermined threshold value and no pre-cool downprocedure is required.

A warm cooling down procedure is typically employed when first startinga newly built apparatus or at the end of a relatively long maintenanceperiod. However, during the lifespan of the apparatus, relatively shortmaintenance operations are to be carried out, at the end of which, therefrigerant is still relatively cold, i.e. well below ambienttemperatures. Instead of waiting for the refrigerant to reach atemperature allowing carrying out a warm cooling down procedure, a coldcooling down procedure is now proposed which makes it possible to cooldown the refrigerant from a relatively cold starting point. This isrelatively time efficient. It also adds flexibility since it allows foran automated cool down for warm and cold conditions.

After the cryogenic heat exchanger has been cooled down with the methodas defined above and/or using the apparatus defined above, thehydrocarbon stream may be liquefied in one or more steps including heatexchanging the hydrocarbon stream in the cryogenic heat exchanger, inorder to produce a liquefied hydrocarbon product.

In another aspect, the invention provides a method of cooling down acryogenic heat exchanger adapted to liquefy a hydrocarbon stream, suchas a natural gas stream, comprising the steps of

-   -   providing a cryogenic heat exchanger arranged to receive the        hydrocarbon stream to be liquefied and a refrigerant, to        exchange heat between the hydrocarbon stream and the        refrigerant, thereby at least partially liquefying the        hydrocarbon stream, and to discharge the at least partially        liquefied hydrocarbon stream and spent refrigerant that has        passed through the cryogenic heat exchanger,    -   providing a refrigerant recirculation circuit to recirculate        spent refrigerant back to the cryogenic heat exchanger, the        refrigerant recirculation circuit comprising at least a        compressor, a compressor recycle valve, a cooler, and a first JT        valve;    -   performing a comparison step (502), the comparison step        comprising:        -   (i) receiving input signals representing sensor signals of            one or more refrigerant temperature indications, providing            an indication of the temperature of the refrigerant,        -   (ii) comparing the one or more refrigerant temperature            indications with one or more associated predetermined            threshold values, and        -   (iii) based on the outcome of the comparison under (ii)            selecting one of an automated warm cooling down procedure of            the cryogenic heat exchanger and an automated cold cooling            down procedure of the cryogenic heat exchanger.

SHORT DESCRIPTION OF THE DRAWINGS

The present invention will now be illustrated by way of example only,and with reference to embodiments and the accompanying non-limitingschematic drawings in which:

FIG. 1 schematically shows a cryogenic heat exchanger arrangementaccording to one embodiment;

FIG. 2 schematically shows a cryogenic heat exchanger arrangementaccording to another embodiment;

FIG. 3 schematically shows a block diagram for automatically coolingdown the cryogenic heat exchanger of FIG. 1 or FIG. 2

FIG. 4 schematically shows a main cryogenic heat exchanger arrangementaccording to another embodiment of the invention as used in a test;

FIG. 5 schematically shows the line-up of FIG. 4 illustrating monitoredtemperatures and pressures;

FIG. 6 shows a block diagram for automatically cooling down thecryogenic heat exchanger of FIG. 4 or FIG. 5; and

FIG. 7 shows a block diagram illustrating some post cool-down task.

DETAILED DESCRIPTION

For the purpose of this description, a single reference number will beassigned to a line (conduit) as well as a stream carried in that line(conduit). Same reference numbers refer to similar components, streamsor lines (conduits).

Described are methods and apparatuses employing a programmablecontroller that is arranged to receive input signals, such as user inputand measurement readings, to process the input signals and producecontrol signals, such as data output and valve control signals.

The programmable controller may be formed as a computer comprising aninput/output device for receiving/transmitting signals, a memoryarranged to store data and a processor arranged to communicate with theinput/output device and memory (reading, writing). The processor isarranged to read and execute program lines, e.g. stored in the memory,to perform the method as described. The memory may also be (partially)located as a separate unit which is accessible by the programmablecontroller.

The programmable controller may be embedded in a distributed controlsystem (DCS), wherein for instance modules provide output via aninterface server, such as an OLE (object-linking and embedding) forprocess control (OPC) that may communicate between the computer programand various interface blocks that may be present in the DCS. In such anarrangement, the DCS can take back control without waiting for theprogrammable controller to transfer control as may be desired duringemergencies or the like.

Automated cooling down of a cryogenic heat exchanger advantageouslyfacilitates cooling down the cryogenic heat exchanger at the highestrate possible without exceeding the specified maximum rate oftemperature change. When cooling down the cryogenic heat exchanger undermanual control, an operator typically has to maintain a wider marginbetween the rate of temperature change and the specified maximum.

Moreover, thanks to the automation as described in this document, aneven more time-efficient cooling down is provided.

Moreover, the methods and apparatuses disclosed herein may also be usedto avoid one or more spatial temperature gradients in or around thecryogenic heat exchanger to exceed a recommended maximum value(s).

The advantages of the methods and apparatuses described herein are morepronounced for cooling down counter-current cryogenic heat exchangers,preferably using an external refrigerant, wherein the evaporatingrefrigerant flows counter-currently relative to the stream or streamsthat is/are to be cooled in the cryogenic heat exchanger against theevaporating refrigerant, than for cooling down co-current cryogenic heatexchangers.

As will be appreciated by the person skilled in the art, the maximumtemperature rate of change and/or maximum spatial temperature gradientis generally dependent on the type and/or specific design of the heatexchanger that is subject to the process of cooling down. Specificrecommendations regarding such values may be provided by themanufacturer.

Where the cryogenic heat exchanger comprises a shell side forevaporating refrigerant and a tube side for auto-cooling therefrigerant, the selected spatial temperature gradient may reflect thetemperature differential between a shell side of the cryogenic heatexchanger and a refrigerant-containing tube side.

There are other preferred temperature gradients to be used, for instancein line-ups wherein downstream of the cooler and upstream of the firstJT valve a liquid/vapour separator is provided in the refrigerantrecirculation circuit, to receive a partly condensed refrigerant andseparate the partly-condensed refrigerant stream into a liquid heavyrefrigerant fraction and a gaseous light refrigerant fraction and todischarge the liquid heavy refrigerant fraction via a liquid outlet andthe gaseous light refrigerant fraction via a gas outlet, which fractionsare passed to the cryogenic heat exchanger, wherein the first JT valveis arranged to control passage of one of these fractions, preferably thelight refrigerant fraction.

The selected spatial temperature gradient may in such a line-up reflectone or more of: the temperature differential between the spentrefrigerant and the refrigerant between the gas outlet and the gaseousrefrigerant inlet of the cryogenic heat exchanger; and the temperaturedifferential between spent refrigerant and the refrigerant between theliquid outlet and the liquid refrigerant inlet of the cryogenic heatexchanger.

Other possible controlled variables include variables indicative ofoperating conditions of one or more compressors, such as surgeconditions. A so-called surge deviation parameter may be determinedbased on sensor data to quantify the deviation between surge and actualoperating condition of the compressor. Typical sensor data that is takeninto account for determining the surge deviation parameter includes theflow through the relevant compressor stage and inlet and dischargepressure of the relevant stage.

For automatically cooling a cryogenic heat exchanger, the one or moremanipulated variables may comprise one or both of: a first JT valvesetting that represents a measure of amount of opening of the first JTvalve; and a compressor recycle valve setting that represents a measureof amount of closing of the compressor recycle valve. The amount ofopening of the first JT valve quite directly affects the rate of coolingof the cryogenic heat exchanger because it is one of the factors thatdetermine the Joule-Thomson effect that the JT valve has on therefrigerant stream as it passes through the JT valve, which determinesthe cooling power of the refrigerant. The amount of closing of thecompressor recycle valve also affects the rate of cooling of thecryogenic heat exchanger because it also influences the JT effect at thefirst JT valve because it is one way of controlling the pressure andflow rate of the refrigerant.

Of course, there are other manipulated variables that can control thepressure and/or flow rate of the refrigerant, such as compressor speed.Thus compressor speed may also be used as one of the manipulatedvariable(s). However, in contrast to speed, a valve is a very suitableitem to manipulate in a control sequence that has relatively immediateeffect on the pressure.

The methods and apparatuses disclosed herein may be used in a method ofliquefying a hydrocarbon stream such as a natural gas stream. In such acase, the cooling down of the cryogenic heat exchanger is followed bynormal operation wherein the hydrocarbon stream is cooled in thecryogenic heat exchanger until it is liquefied, preferably followed bysub-cooling in the cryogenic heat exchanger or in a subsequent heatexchanger.

It is desirable to liquefy a natural gas stream for a number of reasons.As an example, natural gas can be stored and transported over longdistances more readily as a liquid than in gaseous form, because itoccupies a smaller volume and does not need to be stored at a highpressure.

Usually natural gas, comprising predominantly methane, enters an LNGplant at elevated pressures and is pre-treated to produce a purifiedfeed stock suitable for liquefaction at cryogenic temperatures. Thepurified gas is processed through a plurality of cooling stages usingheat exchangers to progressively reduce its temperature untilliquefaction is achieved. The liquid natural gas is then optionallyfurther cooled, and expanded through one or more expansion stages tofinal atmospheric pressure suitable for storage and transportation. Theflashed vapour from each expansion stage can be used as a source ofplant fuel gas.

It is remarked that US 2006/0213223 A1 discloses a liquefaction plantand method for producing liquefied natural gas. Control of the plant maybe fully or partially automated, such as by using an appropriatecomputer, a programmable logic circuit (PLC), using closed-loop andopen-loop schemes, using proportional, integral, derivative (PID)control. However, US 2006/0213223 does not teach a computer program oran algorithm as described in the present application.

As schematically shown in FIG. 1, there is provided a cryogenic heatexchanger 1 arranged to receive, via conduit 2 and hydrocarbon streaminlet 7, the hydrocarbon stream that is to be liquefied, in order toexchange heat between the hydrocarbon stream and an at least partlyevaporating refrigerant 3. As a result of the heat exchanging, thehydrocarbon stream may be at least partially liquefied. The preferablyat least partially liquefied hydrocarbon stream is discharged viahydrocarbon stream outlet 8 into conduit 4. In the embodiment as drawn,conduit 2 and conduit 4 connect via a tube side 29. However, other typesof heat exchangers are possible.

The cryogenic heat exchanger 1 comprises a refrigerant inlet 5 for anexternal refrigerant and a refrigerant outlet 6 for spent refrigerantthat has passed through the cryogenic heat exchanger. A refrigerantrecirculation circuit 10 is provided to recirculate spent refrigerantback to the inlet 5. The refrigerant recirculation circuit 10 comprises,at least, a compressor 11, a compressor recycle valve 12, a cooler 13,and a first Joule-Thompson (first JT) valve 14.

In practical embodiments of the invention, a JT valve may be used incombination with an expander. However, in particular during the coolingdown of the heat exchanger, the JT valve is preferably used forcontrolling the cooling.

In practical embodiments of the invention, the compressor may consist ofa plurality of compression stages, for instance 15 compression stages ormore. A number of these stages, for instance 15 of these stages, may beprovided in the form of an axial compressor or centrifugal compressor inone casing. Each stage may comprise a dedicated recycle valve, and/or asingle recycle valve may be shared by any number of subsequent stages.Several compressors or compressor casings may be arranged in series oneafter another to form a compressor train. Each casing (or compressorstage) may be followed by any number of optional coolers (orintercoolers), and optional knock-out drums to remove any liquid fromthe compressed vapour before passing the compressed vapour to the nextcompression stage. After the last compression stage, the compressedrefrigerant stream may be cooled.

However, for the purpose of illustrating the present invention, aschematically simplified compressor line-up is depicted in FIGS. 1 and2, with only one compressor drawn in and one recycle valve. An examplecomprising two compression stages will be described in more detail withreference to FIGS. 4-6.

In operation, spent (at least partly evaporated) refrigerant is drawnfrom the heat exchanger 1 via outlet 6, and at least a part of it ispassed to a suction inlet of compressor 11 via conduit 25.

The gaseous part of the spent refrigerant stream in conduit 25 iscompressed to yield a compressed refrigerant stream 16 that issubsequently cooled in one or more coolers, here depicted as cooler 13,thereby at least partially condensing the compressed refrigerant stream16 to form an at least partially condensed refrigerant stream 17. The atleast partially condensed refrigerant stream 17 is expanded over firstJT valve 14 and subsequently led into the heat exchanger 1 via inlet 5.

As shown in FIG. 1, the refrigerant stream flows co-currently with thehydrocarbon stream (from left to right) through the heat exchanger 1.However, the flow may be arranged counter-currently instead, such as isfor example the case in FIG. 2.

In FIG. 2 an alternative cryogenic heat exchanger arrangement is shownthat comprises the same elements as the embodiment of FIG. 1, and inaddition includes a refrigerant tube side 15 for auto-cooling therefrigerant. Both the hydrocarbon stream 2 and the refrigerant are heatexchanged against the evaporating refrigerant in the heat exchanger 1.The compressed refrigerant stream 16 is subsequently cooled in one ormore coolers, here depicted as cooler 13, followed by cooling in theheat exchanger 1, via tube side 15, thereby at least partiallycondensing the compressed refrigerant stream 16 to form the at leastpartially condensed refrigerant stream 17. The auto-cooled, at leastpartially condensed refrigerant stream 17, is drawn from the heatexchanger at outlet 18 and led through first JT valve 14 before it ispassed, via inlet 5, into the heat exchanger 1, where it is allowed toat least partially evaporate.

Optionally, a refrigerant make-up system may be provided which iscapable of changing the inventory of the refrigerant in particular inthe case of a mixed refrigerant.

The current invention relates to an apparatus or method for cooling downa cryogenic heat exchanger adapted to liquefy a hydrocarbon stream 2, 7,29, 8, 4, such as a natural gas stream, which cryogenic heat exchanger 1is arranged to receive the hydrocarbon stream to be liquefied and arefrigerant, to exchange heat between the hydrocarbon stream and therefrigerant, thereby at least partially liquefying the hydrocarbonstream, and to discharge the at least partially liquefied hydrocarbonstream and spent refrigerant that has passed through the cryogenic heatexchanger, the apparatus comprising

-   -   a refrigerant recirculation circuit to recirculate spent        refrigerant back to the cryogenic heat exchanger, the        refrigerant recirculation circuit comprising at least a        compressor 11, a compressor recycle valve 12, a cooler 13, and a        first JT valve 14.

The refrigerant recirculation circuit may circulate a single componentrefrigerant, such as methane, ethane, propane, or nitrogen; or amulti-component mixed refrigerant, sometimes referred to simply as mixedrefrigerant (MR), based on two or more components. These components maypreferably be selected from the group comprising nitrogen, methane,ethane, ethylene, propane, propylene, butane and pentane.

The refrigerant circuit may involve any number of separate lines orstreams of refrigerant to cool different hydrocarbon streams, and anynumber of common elements or features, including compressors, coolers,expanders, etc. Some refrigerant streams may be common and some may beseparate.

In a particular embodiment of the present invention, the describedmethod of cooling down a cryogenic heat exchanger is part of a method ofliquefying a hydrocarbon stream such as natural gas from a feed stream.Likewise, the apparatus as described herein may be used in such a methodof liquefying a hydrocarbon stream.

The hydrocarbon stream may be any suitable hydrocarbon-containing,preferably methane-containing, stream to be liquefied, but is usuallydrawn from a natural gas stream obtained from natural gas or petroleumreservoirs. As an alternative, the natural gas stream may also beobtained from another source, also including a synthetic source such asa Fischer-Tropsch process.

Usually natural gas is comprised substantially of methane. Preferablythe feed stream comprises at least 60 mol % methane, more preferably atleast 80 mol % methane.

A hydrocarbon feed stream may be liquefied by passing it through anumber of cooling stages. Any number of cooling stages can be used, andeach cooling stage can involve one or more heat exchangers, as well asoptionally one or more steps, levels or sections. Each cooling stage mayinvolve two or more heat exchangers either in series, or in parallel, ora combination of same.

Various types of suitable heat exchangers able to cool and liquefy ahydrocarbon feed stream are known in the art and the present inventionmay be applied to any one of them. Examples of such heat exchanger typesare heat exchangers available from Air Products & Chemicals Inc. andLinde AG, typically comprising one, or two, or three, or more bundles.

Various arrangements of suitable heat exchangers able to cool andliquefy a feed stream such as a hydrocarbon stream such as natural gasare known in the art, including single mixed refrigerant (SMR)arrangements, dual mixed refrigerant (DMR) arrangements, propane-mixedrefrigerant arrangements (C₃-MR), arrangements based on three or morecycles, such as e.g. a so-called APX arrangement launched by AirProducts & Chemicals Inc. based on C₃-MR—N₂ cycles, and cascadearrangements including those with a sub-cooling cycle. The presentinvention may be applied to any heat exchanger in any of sucharrangements, and other suitable arrangements, with some minormodifications that are within the reach of the person skilled in theart.

In various arrangements, the cooling and liquefying of the hydrocarbonfeed stream involves two (or more) cooling stages comprising apre-cooling stage and a main cooling stage. Typically, the pre-coolingstage cools the hydrocarbon stream to below 0° C., for instance to atemperature in the range −10° C. to −35° C., and the second stage, whichmay be referred to as a main cryogenic stage from −10° C. to −35° C.down to −145° C. to −160° C. or even −170° C. to liquefy the hydrocarbonstream.

The present invention may involve one or more other or furtherrefrigerant circuits, for example in a pre-cooling stage. Any other orfurther refrigerant circuits could optionally be connected with and/orconcurrent with the refrigerant circuit for cooling the hydrocarbonstream.

As indicated above, according to the present embodiments, an apparatusand method are provided for cooling down the heat exchanger 1. Thiscooling down is needed before operating the heat exchanger to actualliquefy the hydrocarbon stream. The cooling down procedures may becontrolled by a programmable controller. Depending on the temperature ofthe refrigerant different cooling down procedures can be employed.

FIG. 3 schematically shows a block scheme representing the steps thatmay be carried out. After a start signal is generated in step 501 (seeFIG. 3), a comparison step 502 is performed which comprises:

(i) receiving one or more refrigerant temperature indications, providingan indication of the temperature of the refrigerant,

(ii) comparing the one or more refrigerant temperature indications withone or more associated predetermined threshold values, and

(iii) based on the outcome of the comparison under (ii), selecting oneof an automated warm cooling down procedure 503 of the cryogenic heatexchanger or an automated cold cooling down procedure 504 of thecryogenic heat exchanger.

Step 502 may comprise a condition check, such as checking whether anappropriate start signal is generated by the distributed control system(DCS) and/or checking if a heartbeat signal is present, i.e. checking ifall the relevant software modules are still active.

Step (i) may comprise obtaining one or more refrigerant temperatureindications comprising at least one of a refrigerant temperatureindication of the refrigerant

-   -   at a suction side of the JT valve 14;    -   at a discharge side of the JT valve 14;    -   at an entry side of the cryogenic heat exchanger 1;    -   at a point inside the cryogenic heat exchanger 1;    -   at a discharge side of the cryogenic heat exchanger 1.

For each received refrigerant temperature indication, one or moresuitable temperature sensors may be provided, producing an indication ofthe temperature of the refrigerant at that location.

Refrigerant temperature indications can be obtained by performingtemperature measurements on the refrigerant directly.

Step (i) may further comprise obtaining an indication of the temperaturedifference between shell side and tube side of the heat exchanger and/ora bottom temperature of the heat exchanger. These temperatures may beused throughout the procedure. All actions may have a condition check,including a temperature difference check.

For each received refrigerant temperature indication, a predeterminedthreshold value is available and each received refrigerant temperatureindication is compared to the associated threshold value. Comparingincludes determining if the received value is above or below thethreshold value.

For instance, the temperature of the refrigerant at the entry side ofthe cryogenic heat exchanger can be compared to a threshold value of−50° C. to determine if the temperature is above or below −50° C.Alternatively, the predetermined threshold value may have any suitablevalue, e.g. −80° C. or −130° C.

If the temperature is below the threshold value the automated coldcooling down procedure 504 is selected and if the temperature is abovethe threshold value the warm cooling down procedure 503 is selected.

According to a further example, the temperature of the refrigerant canbe compared to a first and second threshold value to determine if thetemperature is between the first and second threshold value. Forinstance, the temperature of the refrigerant at the discharge side ofthe cryogenic heat exchanger can be compared to a first threshold valueof −15° C. and to a second threshold value of −55° C. to determine ifthe temperature is between −15° C. and −55° C. or not. This is done toprevent too high temperature differences from occurring between therefrigerant and the heat exchanger. Of course, the exact values dependon the type of heat exchanger that is used.

The warm cooling down procedure will not be described in detail here.Reference is made to WO2009/098278 which provides a detailed explanationof the warm cooling down procedure. The warm cooling down procedurecomprises similar steps as the cold cooling down procedure, but the warmand cold cooling down procedures are not identical, as will be explainedin more detail below.

First action of the cold cooling down procedure is an initial conditionsdefinition action 505 in which the initial conditions are defined. Thisaction may use information on critical and non-critical initialconditions, which may be stored on the memory accessible by theprogrammable controller.

In case of occurrence of a critical condition, the programmablecontroller interrupts the procedure. The procedure may be resumed and/orrestarted after the critical condition has been resolved oracknowledged, and the initial conditions have been acknowledged by anoperator, either manually or by running an automated control procedureto restore the initial condition. In case of a non-critical initialcondition, a warning may be issued. This action 505 may further initiatethe monitoring of critical variables. Only once all critical variablesare within predetermined ranges, the next action (initial opening step506) is commenced.

Examples of critical initial conditions are:

-   -   first JV valve 14 is not sufficiently closed (e.g. more than        0.1% open or other suitable number);    -   pressure in the refrigerant circuit is lower than the compressor        11 discharge;    -   compressor 11 is not on-line and running, as can be determined        by measuring compressor speed (e.g. compressor running at least        3400 rpm or other suitable speed) and verifying that the suction        and discharge valves on the compressors are open;    -   refrigerant pressure is too high (e.g. above 20 barg, or other        suitable figure);    -   compressor inlet guide vane (IGV) is open.

Examples of non-critical initial conditions are:

-   -   various actual temperatures, e.g. temperature of the refrigerant        directly upstream of and directly downstream of the first JV        valve 14, and/or temperature differentials;    -   compressor recycle valves are not fully open (e.g. less than 99%        open or any other suitable value); and    -   compressed refrigerant pressure below a pre-determined minimum        value (as this may unnecessarily slow down the cool-down        processes). A typically suitable minimum value is 18 barg.

The warm and cold cooling down procedure comprise an initial openingstep 506, the initial opening step 506 comprises imposing an initialopening of the first JT valve 14, wherein the initial opening step ofthe first JT valve 14 according to the automated warm cooling downprocedure 503 differs from the initial opening step of the first JTvalve 14 according to the automated cold cooling down procedure 504.

According to an embodiment the initial opening of the first JT valve isgreater in the automated warm cooling down procedure 503 than in theautomated cold cooling down procedure 504.

For instance, the initial opening imposed on the JT valve 14 accordingto the automated cold cooling down procedure 503 may be in the range of1-2%, while the initial opening imposed on the JT valve according to theautomated warm cooling down procedure 504 may be in the range of 3-5%.

In the warm cooling down procedure 503 the JT valve 14 is initiallyopened relatively much, as to check if a Joule-Thompson effect isactually present.

The opening of the valve is expressed in %, which indicates the relativeposition of the valve plug (moveable part of the valve) with respect toits valve seat (stationary part of the valve). As will be understood, 0%means that the valve is fully closed (valve plug against valve seat),100% means that the valve is fully opened (valve plug farthest away fromvalve seat). It will understood that the relation between the valveopening [%] and the flow rate depend on the type of valve used (e.g.ball valve, butterfly valve, linear globe type of valve, fast openingglobe type of valve) and may thus differ from a 1:1 relation.

According to an alternative embodiment the initial opening step 506 ofthe first JT valve 14 in the automated warm cooling down procedure 503comprises imposing a predetermined initial opening of the first JT valve14 (e.g. 3-5%), wherein the initial opening step 506 of the first JTvalve 14 in the automated cold cooling down procedure 504 comprisesdetermining a current opening of the first JT valve 14 and imposing thedetermined current opening of the first JT valve 14.

This allows starting the cold cooling down procedure without adjustingthe setting of the first JT valve. In any case, the predeterminedinitial opening of the cold cooling down procedure is smaller than thepredetermined initial opening of the warm cooling down procedure.

According to an embodiment the initial opening step 506 of the coldcooling down procedure 504 further comprises opening the compressorrecycle valve 12.

This forms a difference with the warm cooling down procedure 503 whereinthe compressor recycle valve 12 remains closed or is actively closed inthe initial opening step of the warm cooling down procedure. In the coldcooling down procedure, the compressor recycle valve 12 may already beopened during the initial opening step 506 of the cold cooling downprocedure 504.

Opening the compressor recycle valve as part of the initial opening step506 will be done mainly in case of a minor trip. Usually the compressorrecycle valve will be closed in the initial opening step 506.

According to an embodiment the programmable controller is arranged to,as part of the initial opening step 506, perform a TROC step comprisingadjusting the opening of the first JT valve 14 based on a determinedtemperature rate of change (TROC) of the refrigerant over the first JTvalve 14 in accordance with an adjustment scheme, wherein the automatedwarm cooling down procedure 503 and the automated cold cooling downprocedure 504 comprise different adjustment schemes.

Both the warm and cold cooling down procedures 503, 504 comprisecomparable TROC steps, but both TROC steps use different conditions todecide on how to adjust the opening of the JT valve 14.

According to an embodiment determining the temperature rate of change(TROC) of the refrigerant over the first IT valve 14 is done bycomparing two refrigerant temperature indications obtained at arespective first t₁ and second t₂ moment in time, the first and secondmoments in time being separated by a predetermined time interval,wherein the predetermined time interval according to the cold coolingdown procedure 504 is shorter than the predetermined time intervalaccording to the warm cooling down procedure 503.

The time interval according to the cold cooling down procedure may beless than 50% of the time interval according to the warm cooling downprocedure.

The time interval according to the cold cooling down procedure maytypically be 2 minutes, while the time interval according to the warmcooling down procedure may typically be 5 minutes. So, according to thisexample, the TROC according to the warm cooling down procedure 503 iscalculated as follows: TROC_(warm)(t)=(T_(t-5)−T_(t))*12[° C./h], wherethe TROC according to the cold cooling down procedure 504 is calculatedas follows: TROC_(cold)(t)=(T_(t-2)−T_(t))*30[° C./h], wherein t_(i) istime in minutes and T is temperature.

The determined TROC is compared to a predetermined TROC threshold valueto prevent too rapid cooling. For instance, according to the coldcooling down procedure 503, the predetermined TROC threshold value maybe 28° C. The adjustment scheme may prescribe that if the TROC_(cold) isabove the predetermined TROC threshold value, e.g. above 28° C., thefirst JT valve 14 will be closed with a certain predetermined closingamount (e.g. 0.5%) and a predetermined waiting time is initiated (e.g. 5minutes) before continuing with opening the first JT valve 14 with apredetermined opening amount (e.g. 0.2%), the predetermined closingamount being greater than the predetermined opening amount.

The temperature measurements used to determine the relevant temperaturerate of change (TROC) can be obtained by measuring the temperature ofthe refrigerant at one or more of the following locations:

-   -   at a suction side of the JT valve 14;    -   at a discharge side of the JT valve 14;    -   at an entry side of the cryogenic heat exchanger 1;    -   at a point inside the cryogenic heat exchanger 1;    -   at a discharge side of the cryogenic heat exchanger 1.

According to an embodiment the adjustment scheme of the cold coolingdown procedure comprises waiting a predetermined time interval betweenimposing an initial opening of the first JT valve and initiating theTROC step. The TROC step comprises (as explained above) adjusting theopening of the first JT valve 14 based on a monitored temperature rateof change (TROC) of the refrigerant over the first JT valve 14.

Waiting a predetermined time interval is done to allow pressure to seepthrough. A high pressure difference will cause a high JT effect andfurther opening too fast could cause a TROC which is too high.

The end of the initial opening step 506 can be determined by determiningthe TROC and verify it is less than a predetermined value or fallswithin a predetermined range.

Once the initial opening step 506 is finished the automated cold cooldown procedure comprises performing an adjustment step 507 whichcomprising simultaneously

-   -   adjusting and closing recycle valve (509) and    -   further adjusting the first JT valve (508).

As will be discussed in more detail below with reference to FIGS. 4-6,action 509 may comprise adjusting and closing a plurality of recyclevalves (509) and/or action 508 may comprise further adjusting aplurality of JT valves, in particular a first and second JT valve.

In action 508 the first JT valve 14 is further adjusted. In particularin the embodiment of FIG. 2, strong cooling may cause condensation ofthe refrigerant. Just before condensation occurs, the valve movementsare preferably slowed down, and the moment that condensation is detectedthe valve may be closed partially to avoid too high a cooling rate thatwould otherwise be caused by a sudden increase in flow rate due tocondensation (an increase of 100 tpd (tonnes per day) 10 secs is notuncommon). After condensation is detected, the valve opening may benormalized and continued until the JT effect of the valve opening indiminished. The JT effect may be monitored during the further opening ofthe JT valve, for instance based on a temperature difference between thetemperature of the refrigerant upstream of the JT valve and thetemperature of the refrigerant downstream of the JT valve. An assumptionmay be made that the JT effect is present if the temperature differenceexceeds 8° C.

Condensation may be detected by deferment from one or both of atemperature and flow measurement at the JT valve. For the refrigerantthat flows through the first JT valve 14, the temperature of therefrigerant downstream of the JT valve 14 may be used and/or the flowthrough the JT valve, which in turn may be estimated by determining apressure differential over the JT valve 14.

In preferred embodiments, the JT valve 14 can't be closed further than aminimum opening corresponding to the opening at the start of thismodule.

The changes in JT effect upon further opening of the JT valve may besmall. However, at the same time the refrigerant pressure is increasedas simultaneously action 509 is performed by manipulating the recyclevalve 12 to meet a target surge deviation of the compressor (or numberof compression stages). This module monitors the surge deviation of thecompressor 11, and closes the recycle valve 12 if the surge deviationexceeds a pre-determined maximum deviation. A suitable predeterminedmaximum deviation is 0.3.

If there are multiple recycle valves, e.g. on multiple compressorstages, each recycle valve may be manipulated individually (butsimultaneously) taking into account a dedicated surge deviationparameter for the corresponding stage through which each particularrecycle valve controls the recirculation.

Since the closing of recycle valve 12 affects the compressor suctionpressure, this pressure is preferably monitored to not go below arecommended limit, such as e.g. 1.8 barg. Closing the recycle valvedecreases the suction pressure as well. Therefore, the closing of therecycle valve is made conditional to avoid causing the suction pressureto go below predetermine target value. The objective is to maintain aramp (increase) on the discharge pressure by closing the recycle valvessteadily while monitoring surge deviation. When the surge deviation isbelow the considered minimum level (e.g. 0.1) then the module activityis stopped. However surge deviation is monitored throughout the wholefinal cool down procedure, and the recycle valves closed when allowed bythe surge deviation and the suction pressure is within a predeterminedrange.

When the temperature of the cryogenic heat exchanger 1 has met itsoperating temperature, an end signal is generated. This may be done aspart of actions 508 and 509, each action generating a separate endsignal, or a single end signal may be generated as part of one of theactions or by a separate action (not shown). When the end signal(s)is/are triggered, the programmable controller may end the automated coldcooling down procedure (action 510 in FIG. 3).

End action 510 may fully close the recycle valve 12 as much as possible,provided that the surge deviation does not stop this from occurring. Ifthe surge deviation prevents further closing of the recycle valve, incase the surge value is too low (typically below 0.1), a warning messagemay be generated and outputted to alert the operator that an IGVadjustment may be necessary. An IGV movement has a similar effect as theclosing of the recycle valve 12. However, any IGV movement may beconstrained by the molecular weight of the passing refrigerant that mustexceed a pre-determined minimum value. A typical MR minimum molecularweight is 24 g/mol. Obviously this warning signal may not be a usefuloption if no IGV is present on the compressor in use.

Since an IGV movement is considered to be a last resource, it has beencontemplated to only alert the operator to the possible necessity of anIGV movement instead of attempting to execute any IGV movement under thecontrol of the automatic procedure as described herein.

Once the recycle valve is fully closed or closed sufficiently, controlmay be handed over an operator and/or present a status output orgenerate an operator alerting signal to inform the operator that normaloperation of the cryogenic heat exchanger may proceed, or the like.Alternatively, a subsequent control procedure or the like may bestarted, e.g. normal operating control such as advanced process controlas described in e.g. U.S. Pat. Nos. 7,266,975 and/or 6,272,882 or anyother type of module.

FIG. 4 shows a larger type of cryogenic heat exchanger 100, embedded ina system of various pre-cooling heat exchangers, serviced by such afurther refrigerant circuit, and other equipment, as may be found in ahydrocarbon liquefaction plant. The further refrigerant circuit mayhereinafter be referred to as the “pre-cooling refrigerant circuit” or“pre-cooling refrigerant cycle”. Likewise, items such as compressors andthe refrigerant may also be referred to as “pre-cooling refrigerantcompressor” or “pre-cooling refrigerant”.

The cryogenic heat exchanger 100 of this embodiment will hereinafter bereferred to as the main cryogenic heat exchanger 100, to distinguish itfrom any other heat exchangers present in the embodiment. The maincryogenic heat exchanger 100 comprises a warm end 33, a cold end 50 anda mid-point 27. The wall of the main cryogenic heat exchanger 100defines a shell side 110. In the shell side 110 are located:

-   -   a first tube side 29 extending from the warm end 33 to the cold        end 50, preferably extending between a hydrocarbon stream inlet        7 and a hydrocarbon stream outlet 8;    -   a second tube side 28 extending from the warm end 33, preferably        from a gaseous refrigerant inlet 49 a at the warm end 33, to the        mid-point 27; and    -   a third tube side 15 extending from the warm end 33, preferably        from a liquid refrigerant inlet 49 b at the warm end 33, to the        cold end 50.

A refrigerant compressor train, as shown here symbolically comprisingfirst and second compressors 30 and 31, is provided to compress therefrigerant. Each of these compressors is provided with a number ofrecycle valves, which are here schematically represented by recyclevalves 130 and 131 in a recycle line that connects the compressordischarge, downstream of the respective coolers, to the low pressuresuction inlet.

The first refrigerant compressor 30 is driven by a suitable motor, forexample a gas turbine 35, which is provided with a helper motor 36 forstart-up, and the second refrigerant compressor 31 is driven by asuitable motor, for example a gas turbine 37 provided with a helpermotor (not shown). Alternatively, the compressors 30 and 31 may bedriven on a single shaft on a shared motor.

During normal operation after the main cryogenic heat exchanger has beencooled down, a gaseous, preferably methane-rich hydrocarbon feed streamis supplied at elevated pressure through supply conduit 20 to the firsttube side 29 of the main cryogenic heat exchanger 100 at its warm end33. The hydrocarbon feed stream passes through the first tube side 29where it is cooled, liquefied and optionally sub-cooled, against a mixedrefrigerant (MR) evaporating in the shell side 110 forming spentrefrigerant. The resulting liquefied hydrocarbon stream is removed fromthe main cryogenic heat exchanger 100 at its cold end 50 through conduit40. The flow of the hydrocarbon stream through the system may becontrolled, e.g. using rundown valve 44 provided in conduit 40.

Stream 40 may optionally be passed through a suitable end flash system,wherein the pressure is brought down to storage and/or transportationpressure. Finally, liquefied hydrocarbon stream is passed as the productstream to storage where it is stored as liquefied product, or optionallydirectly to transportation.

During normal operation, and during cooling down of the main cryogenicheat exchanger, spent refrigerant is removed from the shell side 110 ofthe main cryogenic heat exchanger 100 at its warm end 33 through conduit25 and passed to knock-out drum 56.

A refrigerant make-up adjustment conduit 65 also feeds into knock-outdrum 56 to optionally add refrigerant inventory to the spent refrigerantstream. The adding of the various refrigerant components is controlledby one or more valves, typically one valve per component. Here, thesevalves are schematically represented as valve 66.

The evaporated fraction 55 of the spent refrigerant, which exits fromthe top of the knock out drum 56, is compressed, in refrigerantcompressors 30 and 31, to obtain a compressed refrigerant stream, whichis removed through conduit 32. Other refrigerant compressor arrangementsare possible.

In between the two refrigerant compressors 30 and 31, heat ofcompression is removed from the fluid passing through conduit 38 inambient cooler 23, which may comprise an air cooler and/or a watercooler and/or any other type of ambient cooler. Likewise, an intercooler(not shown) may be provided between two successive compressor stages ofa compressor.

The compressed refrigerant stream in conduit 32 is cooled in air cooler42 and partly condensed in one or more pre-cool heat exchangers (shownare 43 and 41) against a pre-cool refrigerant cycle that will bedescribed in more detail later herein below. The pre-cool heatexchangers 41, 43 may be operating at mutually different pressuresand/or be using different refrigerant compositions.

The partly condensed refrigerant stream 39 is then passed to and letinto a liquid/vapour separator via an inlet device, here depicted asseparator vessel 45 and inlet device 46. In the separator vessel 45, thepartly-condensed refrigerant stream is separated into a, at this pointliquid, heavy refrigerant fraction (MIR) and a, at this point gaseous,light refrigerant fraction (LMR). These streams may each be individuallycontrolled by means of a JT valve or the like, the first JT valve 58 forcontrolling the vapour (light) refrigerant stream and a second JT valve51 for controlling the liquid (heavy) refrigerant stream.

The liquid heavy refrigerant fraction is removed from the separatorvessel 45 through conduit 47, and the gaseous light refrigerant fractionis removed through conduit 48. The heavy refrigerant fraction issub-cooled in the second tube side 28 of the main cryogenic heatexchanger 100 to get a sub-cooled heavy refrigerant stream 54. Thesub-cooled heavy refrigerant stream is removed from the main cryogenicheat exchanger 100 through conduit 54, and allowed to expand over anexpansion device comprising second JT valve 51. The expansion device mayfurther comprise a dynamic expander (not shown) in series with thesecond JT valve 51, which does not have to be operated during any cooldown procedure of the main cryogenic heat exchanger.

The sub-cooled heavy refrigerant stream is, at reduced pressure,introduced through conduit 52 and nozzle 53 into the shell side 110 ofthe main cryogenic heat exchanger 100 at its mid-point 27. The heavyrefrigerant stream is allowed to evaporate in the shell side 110 atreduced pressure, thereby cooling the fluids in the tube sides 29, 28and 15.

The gaseous light refrigerant fraction removed from separator vessel 45through conduit 48 is passed to the third tube side 15 in the maincryogenic heat exchanger 100 where it is cooled, liquefied andsub-cooled to get a sub-cooled light refrigerant stream 57. Thesub-cooled light refrigerant stream is removed from the main cryogenicheat exchanger 100 through conduit 57, and allowed to expand over anexpansion device comprising first JT valve 58. At reduced pressure it isintroduced through conduit 59 and nozzle 60 into the shell side 110 ofthe main cryogenic heat exchanger 100 at its cold end 50. The lightrefrigerant stream is allowed to evaporate in the shell side 110 atreduced pressure, thereby cooling the fluids in the tube sides 29, 28and 15.

Optionally (not shown), an optional side stream may be drawn from thegaseous light refrigerant stream 48, which may be cooled, liquefied andsub-cooled against one or more other cold streams in one or more otherheat exchangers other than the main cryogenic heat exchanger 100. Forinstance, it may be cooled, liquefied and sub-cooled against cold flashvapour generated from stream 40 in an optional end flash system. Theoptional sub-cooled side stream may be recombined with the lightrefrigerant stream in conduit 57 or 59 in which case it needs anauxiliary expander means such as an auxiliary first JT valve. Referenceis made to U.S. Pat. No. 6,272,882 for a more detailed description ofsuch an option.

Pre-cool heat exchangers 41,43 are operated using a pre-coolingrefrigerant, which may be a mixed component refrigerant or a singlecomponent refrigerant. For this example, propane has been used.Evaporated propane is compressed in pre-cool compressor 127 driven by asuitable motor, such as a gas turbine 128. A pre-cooling refrigerantcompressor recycling valve 129 is provided as well, here symbolicallyshown in a line connecting the first stage compressor low pressuresuction inlet with the intermediate pressure level. However, a recyclingline may optionally be provided across all of or a selection ofcompression stages.

Compressed propane is then condensed in air cooler 130, and thecondensed compressed propane, at elevated pressure, is then passedthrough conduits 135 and 136 to heat exchangers 43 and 41 which arearranged in series with each other. The condensed propane is allowed toexpand to an intermediate pressure over expansion valve 138, beforeentering into heat exchanger 43. There, the propane partly evaporatesagainst the heat from the multi-component refrigerant in conduit 32, andthe resulting evaporated gaseous fraction is passed through conduit 141to an intermediate pressure inlet of the propane compressor 127. Theliquid fraction is passed through conduit 145 to the heat exchanger 41.Before entering into the heat exchanger 41, the propane is allowed toexpand to a low pressure over expansion valve 148. The evaporatedpropane is passed through conduit 150 to a suction inlet of the propanecompressor 127.

As the person skilled in the art knows, knock-out drums or the like maybe provided in any conduit connecting to a compressor suction to avoidfeeding a non-gaseous phase to the compressor. An economizer may also beprovided.

In the present example, two pre-cooling heat exchangers have been shownoperating at two pressure levels. However, any number of heatpre-cooling heat exchangers and corresponding pressure levels may beemployed.

The pre-cooling refrigerant cycle may also be used to obtain hydrocarbonstream 20, for instance as follows. A hydrocarbon feed, in the presentexample a natural gas feed, is passed at elevated pressure throughsupply conduit 90. The natural gas feed, which typically is amulti-component mixture of methane and heavier constituents, ispartially condensed in at least one heat exchanger 93.

In the present example, this heat exchanger operates at approximatelythe same pressure level as pre-cooling heat exchanger 43, using a sidestream 137 of the pre-cooling refrigerant drawn from conduit 135.Although not drawn in FIG. 4, conduit 137 connects to conduit 137 a.Prior to entering into the heat exchanger 93, the pre-coolingrefrigerant is allowed to expand over valve 139 to approximatelyintermediate pressure. The resulting evaporated gaseous fraction ispassed through conduits 140 a and 140 to conduit 141 where it isrecombined with the gaseous fraction drawn from pre-cooling heatexchanger 43. The liquid fraction of the pre-cooling refrigerant isdrawn from the heat exchanger 93 in conduit 151 and fed into heatexchanger 91 after expansion over valve 152 to approximately the lowpressure. The evaporated pre-cooling refrigerant is then led to conduit150 via conduits 153 a and 153.

It is remarked that heat exchangers 43 and 93 and/or heat exchangers 41and 91 may be provided in the form of combined heat exchangerscomprising separate sides for the natural gas and for the refrigerant inconduit 32.

The partly condensed feed 92 is introduced, e.g. via an inlet device 94,into a gas/liquid separator 95 which may be provided e.g. in the form ofa scrub column or similar. In the scrub column 95, the partly condensedfeed is separated to get a methane-enriched gaseous overhead stream 97and a liquid, methane-depleted bottom stream 115.

The gaseous overhead stream 97 is passed through conduit 97 via heatexchanger 91 to an overhead separator 102. In the heat exchanger 100,the gaseous overhead stream is partly condensed against the pre-coolingrefrigerant in conduit 151, and the partly condensed overhead stream isintroduced into the overhead separator 102 via inlet device 103. In theoverhead separator 102, the partly condensed overhead stream isseparated into a gaseous, stream 20 (which is substantially depletedfrom C5+ components and/or relatively rich in methane when compared tothe feed stream) and a liquid bottom stream 105. The gaseous stream 20forms the hydrocarbon feed at elevated pressure in conduit 20.

At least part of the liquid bottom stream 105 may be introduced throughconduit 105 and nozzle 106 into the scrub column 95 as reflux. Theconduit 105 is provided with a flow control valve (not shown) and/or apump 108.

If there is less reflux required than there is liquid in the partlycondensed gaseous overhead stream 105, the surplus may be passed on toconduit 20 over a bypass conduit (not shown) and a flow control valve(not shown). In case too little reflux is available, an external refluxmedium, suitably butane, may be added from an external source (notshown), suitably into conduit 105.

The liquid, C3+-enriched bottom stream is removed from the scrub column95 via conduit 115. Here it may be withdrawn from the process, sent to afractionation train and/or storage/transport and/or a reboiler in anyfashion known to the person skilled in the art.

Prior to its normal operation as described above, the main cryogenicheat exchanger has to be cooled down to operating temperature. Thepresently disclosed methods and apparatuses achieve an automated coolingdown of the main cryogenic heat exchanger. This has been demonstrated inaccordance with the following.

Several temperatures, temperature rates of change, and temperaturedifferentials at various points in and around the main cryogenic heatexchanger may be monitored by the programmable controller during thecool down process. This enables the programmable controller to determinethe evolution of the temperature profile over time. FIG. 5 shows thepoints in and around the main cryogenic heat exchanger 100 where in atest the temperature sensors (TR20; TR25; TR33; TR40; TR47; TR48; TR52;TR54; TR57; TR59) and differential temperature sensors (TDR2547;TDR2548; TDR2715; TDR5254; TDR5759) were provided in addition to othertemperature and temperature differential sensors that will not befurther discussed here as they were considered of less relevance for thedescribed automation.

The line-up in FIG. 5 corresponds to the line-up of FIG. 4, but thereference numbers have been omitted in the interest of highlighting thereference numbers corresponding to the various sensors that are shown.Temperature sensors are marked by “TR” followed by a number thatcorresponds to the reference number assigned to the component, stream orline (conduit) where the sensor is provided. For temperaturedifferential sensors, the code TDR is used followed by two two-digitnumbers corresponding to the reference numbers assigned to thecomponents, streams or lines (conduits) between which the differentialsensor is provided. The temperature sensors and differential temperaturesensors generate sensor signals that may be received by and monitored bythe programmable controller which may use one or more of these ascontrolled variables.

At the top of the main cryogenic heat exchanger 100, temperatures inconduits 57 and 59, upstream and downstream of the first JT valve 58,were monitored using temperature sensors TR57 and TR59. The differencebetween these temperatures was also monitored, which may be used todetermine the actual JT effect over the first JT valve.

The difference between the shell temperature at mid-point 27 wasmeasured and the temperature in tube side 15 at mid-point 27 wasdetermined (TDR2715). In addition, the shell temperature near the warmend 33 was measured using TR33, as well as the temperature of the spentrefrigerant drawn from the heat exchanger in conduit 25 (TR25).

The inlet temperature of the heavy liquid refrigerant fraction may bemeasured using TR47, inlet temperature of the hydrocarbon streamimmediately upstream of the main cryogenic heat exchanger 100 may bemeasured using TR20, and the temperature of the hydrocarbon rundownstream immediately downstream of the main cryogenic heat exchanger 100may be measured using TR40.

All temperature measurements stabilize and are reliable when there isforward flow. Thus, the measurements can be unreliable at times, forinstance when stagnant gas goes back to the temperature sensor at thebeginning of cool down. The monitoring depends on the initialconditions, pressure conditions for example. The temperature thatindicates the end of the cool down is the hydrocarbon product rundownline temperature TR40. However, this measurement may not be reliable atthe beginning of cool down when the hydrocarbon flow is extremely low.Therefore, at the beginning of cool down another temperature, suitablythe LMR temperature TR59 downstream of the first JT valve 58, may bemonitored instead. However at the end of cool down the referencetemperature will be TR40.

Several pressures and pressure differentials, in various points in theline-up, may be monitored by the programmable controller during the cooldown process. The most relevant pressure sensors (PR32; PR54; PR55;PR57; PR150) are indicated in FIG. 5, using PR followed by a number thatcorresponds to the reference number assigned to the component or line(conduit) where the sensor is provided. The most important pressures tobe monitored include the pre-cool compressor suction pressure PR150 inconduit 150, the mixed refrigerant compressor 30 suction pressure (PR55)in conduit 55; and the mixed refrigerant compressor discharge pressurePR32 in conduit 32.

These pressure sensors generate sensor signals that may be received byand monitored by the programmable controller which may use one or moreof these as controlled variables.

The pressure in the line-up after a long shut down can affect thecooling procedure, especially if the line-up has been in full recyclefor days. Small changes, while having a high pressure, may have bigconsequences in the overall cooling of the main cryogenic heat exchanger100. Additionally, PR57 and PR54 (LMR and HMR tube pressure upstream ofthe first (58) and second (51) JT valves, respectively) may be monitoredbefore cool down. Any valve manipulation may have faster dynamics ifthese pressures are too high, so as initial condition the system shouldhave a pressure level that is lower than a predetermined initial maximumpressure value (in the test we used 20 barg).

Flow rates may be calculated for the LMR and HMR streams, in order to beused as a controlled variable or at least a variable to be monitored.Such calculations may be based on the differential in pressure and thenominal valve opening of the first (58) or second (51) JT valve,respectively. For this, measurements of the pressures before the firstand second JT valves on both LMR and HMR circuits (PR57 and PR54,respectively) and the suction pressure (PR55) of the refrigerant circuitbefore going to the compressors may be used. The standard deviation offlow measurements for small JT valve openings may be quite large, whichcould lead to errors if used as monitored variable. A linear model ofthe LMR and HMR flows has been calculated as the Least Squares Linearmodel from all measurements with high valve openings. Based on thismodel, the estimated flows will be given by:

F _(LMR) =K _(LMR) ·X ₅₈·√(PR57−PR55); and

F _(HMR) =K _(HMR) ·X ₅₁·√(PR54−PR55)

wherein F_(LMR) (F_(HMR)) represents the flow rate in the LMR conduit 48(HMR conduit 47); X₅₈ (X₅₁) represents the amount of opening of thefirst (second) JT valve 58, resp. 51; and K_(LMR) (K_(HMR)) representsthe least squares linear model constant corresponding to the slope. Alinear least squares model has been found to satisfy the desiredaccuracy. However, other types of functions may be employed instead. Inparticular, a quadratic function could be estimated for the HMR, whilefor the LMR flow a characteristic shape resembling a square rootfunction has been found.

Immediately prior to executing the automated cooling down, the maincryogenic heat exchanger 100 was first pre-cooled, under manual control,to a temperature between about −25° C. and about −35° C. Other tasksthat have been completed at this stage, for the time being manually butthese could also be automated and incorporated in the module structureas presently disclosed, include:

-   -   level control in any in-line NGL (natural gas liquid, typically        consisting of molecules having mass comparable to propane and        higher) extraction column (e.g. scrub column);    -   temperature control of stream 20;    -   depressurisation of the refrigerant circuit, notably tube-sides        15, 28;    -   defrosting of gas/cold gas mixture controls, used to cool the        refrigerant circuit tubes to the temperature of between about        −25° C. and about −35° C.

Further cooling down of the main cryogenic heat exchanger to theoperating temperature of below about −155° C., here to an operatingtemperature of about −160° C., was achieved using the automated coolingdown method and apparatus. The further cooling down may hereinafter bereferred to as the “final cool down”.

So, step (i) as described above may comprise obtaining one or morerefrigerant temperature indications comprising at least one of arefrigerant temperature indication of the liquid, heavy refrigerantfraction (MIR) and/or the gaseous, light refrigerant fraction (LMR)

-   -   at a suction side of the JT valve 14;    -   at a discharge side of the JT valve 14;    -   at an entry side of the cryogenic heat exchanger 1;    -   at a point inside the cryogenic heat exchanger 1;    -   at a discharge side of the cryogenic heat exchanger 1.

So, in view of the embodiment described above with reference to FIGS. 4and 5, according to an embodiment the refrigerant recirculation circuitto recirculate spent refrigerant back to the cryogenic heat exchangercomprises a plurality of compression stages with each compression stagecomprising a compressor recycle valve (130, 131) and the adjustment step(507) comprises adjusting and closing the plurality of recycle valves(509 a, 509 b).

So, in view of the embodiment described above with reference to FIGS.4-6, according to an embodiment downstream of the cooler 42 and upstreamof the first JT valve a liquid/vapour separator 45 is provided in therefrigerant recirculation circuit, to receive a partly condensedrefrigerant and separate the partly-condensed refrigerant stream into aliquid heavy refrigerant fraction (MIR) and a gaseous light refrigerantfraction (LMR) and to discharge the liquid heavy refrigerant fractionvia a liquid outlet and the gaseous light refrigerant fraction via a gasoutlet, which fractions are passed to the cryogenic heat exchanger,wherein the first JT valve is arranged to control passage of one ofthese fractions, preferably the light refrigerant fraction and wherein asecond JT valve is arrange to control passage of the other of thesefractions, preferably the heavy refrigerant fraction.

Next, with reference to FIG. 6 a block diagram for automatically coolingdown the cryogenic heat exchanger of FIG. 4 or FIG. 5 will be described.

Similar to the embodiment described above with reference to FIG. 3,after a start signal is generated in step 501, a comparison step 502 isperformed which comprises:

(i) receiving one or more refrigerant temperature indications, providingan indication of the temperature of the refrigerant,

(ii) comparing the one or more refrigerant temperature indications withone or more associated predetermined threshold values, and

(iii) based on the outcome of the comparison under (ii), selecting oneof an automated warm cooling down procedure 503 of the cryogenic heatexchanger or an automated cold cooling down procedure 504 of thecryogenic heat exchanger.

The warm cooling down procedure 503 will not be described in more detailhere. Reference is made to WO2009/098278 in which a detailed descriptionof the warm cooling down procedure is provided.

The cold cooling down procedure 504 starts with defining the initialconditions in action 505 much in the same way as described above.Examples of critical initial conditions are:

-   -   presence of an excess of heavy components in the hydrocarbon        feed (e.g. in line 20) if the hydrocarbon flow is manipulated        (generally a maximum of 0.08 mol % of C5+ is tolerated);    -   first and second JT valves (58, 51) not sufficiently closed (in        the test a value of more than 1% open was used);    -   pressure in refrigeration circuit (LMR and HMR) is lower than        the compressor 31 discharge;    -   one or more of refrigerant compressors 30, 31, and pre-cool        refrigerant compressor 127 is not on-line and running (as e.g.        monitored by compressor speed);    -   suction and discharge valves on these compressors are not open;    -   refrigerant pressure at the compressor 31 discharge is too high        (the test used a maximum of 20 barg);    -   pre-cooling refrigerant compressor 127 suction pressure outside        of a predetermined pressure window (suitably a window around        approximately 0.5 barg);    -   any IGV valve present is not sufficiently closed.

Examples of non-critical initial conditions are:

-   -   TDR5759 too small (a typical minimum value recommended in case        of a coil wound heat exchanger from Air Products & Chemicals Inc        is 25° C.);    -   one or more of refrigerant compressor recycle valves (e.g. 130,        131) are not sufficiently open (the test used less than 99%        open);    -   discharge pressure of compressor 31 below a pre-determined        minimum value (the test used 18 barg).

When the initial conditions are defined and possible warnings areresolved, actions 506 and 511 are triggered.

In initial opening action 506 the first JT valve 58 for controlling thevapour (light) refrigerant stream and a second JT valve 51 forcontrolling the liquid (heavy) refrigerant stream are set at an initialopening (506 a), wherein the initial opening according to the automatedwarm cooling down procedure 503 differs from the initial opening step ofthe first JT valve 14 according to the automated cold cooling downprocedure 504. Preferably, the initial openings of the JT valves aregreater in the automated warm cooling down procedure 503 than in theautomated cold cooling down procedure 504.

Action 506 b, following action 506 a initiates a waiting time asdescribed above.

The initial opening step may comprise imposing an initial opening of thefirst and second JT valve, wherein the initial opening step of the firstand second JT valves (51, 58) according to the automated warm coolingdown procedure differs from the initial opening step of the first andsecond JT valves (51, 58) according to the automated cold cooling downprocedure.

In particular, the initial opening of the first and second JT valves isgreater in the automated warm cooling down procedure than in theautomated cold cooling down procedure.

The initial opening step may further comprise performing a TROC stepcomprising adjusting the opening of the first and second JT valves 51,58 based on a determined temperature rate of change (TROC) of therefrigerant over the first and second JT valves 51, 58 in accordancewith an adjustment scheme, wherein the automated warm cooling downprocedure 503 and the automated cold cooling down procedure 504 comprisedifferent adjustment schemes.

In particular, the adjustment scheme of the cold cooling down proceduremay comprise waiting a predetermined time interval between imposing aninitial opening of the first and second JT valves 51, 58 and initiatingadjusting the opening of the first and second JT valves 51, 58 based ona monitored temperature rate of change (TROC) of the refrigerant overthe first and second JT valves 51, 58.

Next, after action 506 has been completed the automated cold coolingdown procedure comprises performing an adjustment step (507) comprisingsimultaneously

-   -   adjusting and closing recycle valve (509) and    -   further adjusting the first and second JT valves (508 a, 508 b).

As described above, there may be a plurality of compression stages witheach compression stage comprising a compressor recycle valve (130, 131)and action 509 may thus comprise adjusting and closing a plurality ofrecycle valves 130, 131.

The make-up adjustment is controlled in action 512 which is performedparallel to action 507 and manipulates the make-up to:

-   -   Increase the compressor 31 discharge pressure along a ramp        towards a target operating pressure (in the test, 30 barg);    -   Move the refrigerant composition towards a target composition,        which may be an end target for normal operation of the main        cryogenic heat exchanger 100 or an intermediate target.

The refrigerant target composition may change during the cool downprocedure. It may change gradually or step wise upon a controlledvariable reaching a predetermined value. For instance, it may changeonce the temperature TR57 goes below a predetermined value of −135° C.or −140° C.

Parallel to actions 506 and 507, action 511 is executed which adjustsone or more of the pre-cool refrigerant compressor recycle valve(s),here in the form of the first stage recycle valve 129 that controlsrecycle stream through the first compression stage of compressor 127.The module objective is to maintain a suction pressure on the pre-coolrefrigerant suction pressure (in conduit 150 of FIG. 4) within apre-determined range, e.g. 0.25-0.50 barg, but without reducing thesurge deviation too close to the control line. The low pressure willassure that the temperature of the hydrocarbon feed gas going into themain cryogenic heat exchanger 100 (e.g. via conduit 20) has a reasonablevalue. Therefore, the temperature in conduit 20 itself does not need tobe monitored or used as condition for control in this module.

Additionally, the pre-cooling refrigerant compressor 127 dischargetemperature (in conduit 135) was not monitored, since the automated cooldown procedure as used in the test did not offer a capability tomanipulate any variable that could be used to improve the situation of ahigh discharge temperature of the pre-cooling refrigerant compressor127. However, this may be implemented without departing from the scopeof the invention.

There may be built in some overriding boundaries, for one or more of themonitored variables. Crossing of one of these boundaries (i.e. exceedinga pre-determined maximum and/or minimum value) by one or more of themonitored variables may result in issuance of a warning signal to alertan operator, or pausing the cooling down, or abortion of the coolingdown, or a combination of these.

Typical examples of such overriding boundaries are:

-   -   a pre-determined maximum temperature rate of change (e.g. 28°        C./hour as specified for an Air Products cryogenic heat        exchanger) on any selected temperature, suitably one or more of        a temperature of the hydrocarbon product at a location in tube        side 29 and/or in the discharge conduit 40; the spent        refrigerant temperature (e.g. in bottom warm end of the shell        side 33 or in conduit 25); the refrigerant temperature at the        discharge side of the first JT valve 58 or the second JT valve        51, or at the suction side thereof; any shell side temperature        in the heat exchanger 1;    -   a pre-determined maximum spatial temperature gradient,        reflecting a specified maximum temperature difference between        two spatially separated points in or around the heat exchanger        (e.g. a maximum temperature difference of 28° C.), suitably the        temperature difference TDR2547 between the light refrigerant        upstream of main cryogenic heat exchanger 100 and the spent        refrigerant (also possible: TDR3347, not shown); the temperature        difference TDR2548 between the heavy refrigerant upstream of        main cryogenic heat exchanger 100 and the spent refrigerant        (also possible: TDR3348, not shown); TDR2715; and TDR5759;    -   a predetermined maximum content (0.08 mol %) of heavy components        in the hydrocarbon feed stream that would freeze in the main        cryogenic heat exchanger 100;    -   suction and discharge valves on the refrigerant compressors        closed;    -   a maximum specified top shell pressure (5 barg) at the cold end        of the main cryogenic heat exchanger;    -   detection of a trip;    -   existence of communication errors in the control system.

Clearly, other overriding boundaries may be used, e.g. in case of othertypes of cryogenic heat exchangers being used.

Although not implemented in the test, it has been contemplated tofurther embed the above block diagrams (FIG. 6 or a similar one foranother line-up or heat exchanger) in a larger structure comprisingother, preceding or subsequent actions, or both. An example is shown inFIG. 7.

FIG. 7 shows an example with some post cool-down tasks. These may, forinstance, be intermediate tasks that need to be completed before anautomatic process control system for normal operation can take over thecontrol. For instance, module 401 manipulates the run down valve 44,with the goal to ramp up the flow through conduit 20 and 40 and thehydrocarbon tube side 29.

Other modules could therefore be in parallel to module 401. As anexample, module 402 has been depicted, but also included could be amodule for ramping up any fractionation section that may be provideddownstream of any NLG extraction column to receive and furtherfractionate the extracted NLG liquids. The person of skill in the artwould be able to work out which manipulated and controlled variablescould be used, depending on the type of line-up and equipment used.

The apparatuses and methods described herein may be applied to cryogenicheat exchangers whenever a cryogenic heat exchanger needs to be cooleddown before operation. This could for instance be initial cooling down,or cooling down after a maintenance operation or after a trip: thereason why the heat exchanger was warmer than operation temperature isnot material to the application of the subject matter described herein.

The person skilled in the art will understand that the present inventioncan be carried out in many various ways without departing from the scopeof the appended claims. The invention has been described withparticularity, including providing target values for certain controlledvariables. However, it will be apparent to the person skilled in the artthat these values were chosen in connection to the specific line up andequipment used for the test. Such details may need to be optimized whenthe invention is to be carried out on another line-up using otherequipment, and therefore should not be considered as limiting the scopeof the present invention.

What is claimed is:
 1. A method of cooling down a cryogenic heatexchanger adapted to liquefy a hydrocarbon stream, the method comprisingthe steps of receiving, by the cryogenic heat exchanger, the hydrocarbonstream to be liquefied and a refrigerant, to exchange heat between thehydrocarbon stream and the refrigerant, thereby at least partiallyliquefying the hydrocarbon stream, and to discharge the at leastpartially liquefied hydrocarbon stream and a spent refrigerant that haspassed through the cryogenic heat exchanger, recirculating, by arefrigerant recirculation circuit, the spent refrigerant back to thecryogenic heat exchanger, wherein the refrigerant recirculation circuitcomprises at least a compressor, a compressor recycle valve, a cooler,and a first Joule Thomson (JT) valve; performing a comparison step by aprogrammable controller, said comparison step comprising: (i) receivingone or more refrigerant temperature indications, and providing anindication of a temperature of the refrigerant, (ii) comparing the oneor more refrigerant temperature indications with one or more associatedpredetermined threshold values, and (iii) based on an outcome of thecomparison under (ii) selecting one of an automated warm cooling downprocedure of the cryogenic heat exchanger and an automated cold coolingdown procedure of the cryogenic heat exchanger; wherein the automatedcold cooling down procedure is adapted to reduce a temperature of thecryogenic heat exchanger from a cold condition down to LNG productionpoint and is allowed to start with an opened first JT valve, wherein theautomated warm cooling down procedure comprises an initial opening step,wherein the initial opening step comprises imposing an initial openingof the first JT valve, wherein the initial opening step of the first JTvalve according to the automated warm cooling down procedure differs insize of the opened first JT valve according to the automated coldcooling down procedure; and performing a TROC step by the programmablecontroller, the TROC step comprising adjusting the opening of the firstJT valve based on a determined temperature rate of change (TROC) of therefrigerant over the first JT valve in accordance with an adjustmentscheme, wherein the automated warm cooling down procedure and theautomated cold cooling down procedure comprise a plurality of differentadjustment schemes.
 2. The method of claim 1, further comprising thesteps of: subsequently liquefying the hydrocarbon stream in one or moresteps including at least heat exchanging the hydrocarbon stream in thecryogenic heat exchanger.
 3. The method according to claim 1, whereinthe one or more refrigerant temperature indications comprise at leastone of a refrigerant temperature indication of the refrigerant at asuction side of the first JT valve; at a discharge side of the first JTvalve; at an entry of the cryogenic heat exchanger; at a point insidethe cryogenic heat exchanger; at a discharge side of the cryogenic heatexchanger.
 4. The method according to claim 1, wherein the initialopening of the first JT valve of the automated warm cooling downprocedure is greater than the opening of the opened first JT valve ofthe automated cold cooling down procedure.
 5. The method according toclaim 1, wherein the initial opening step of the first JT valve in theautomated warm cooling down procedure comprises imposing a predeterminedinitial opening of the first JT valve, and wherein the automated coldcooling down procedure comprises determining a current opening of thefirst JT valve and imposing the determined current opening of the firstJT valve.
 6. The method according to claim 1, wherein the cold coolingdown procedure comprises opening the compressor recycle valve.
 7. Themethod according to claim 1, wherein determining the temperature rate ofchange (TROC) of the refrigerant over the first JT valve is done bycomparing two refrigerant temperature indications obtained at arespective first and second moment in time, the first and second momentin time being separated by a predetermined time interval, wherein thepredetermined time interval according to the cold cooling down procedureis shorter than the predetermined time interval according to the warmcooling down procedure.
 8. The method according to claim 5, wherein theadjustment scheme of the cold cooling down procedure comprises waiting apredetermined time interval between imposing the determined currentopening of the first JT valve and initiating adjusting the opening ofthe first JT valve based on the monitored temperature rate of change(TROC) of the refrigerant over the first JT valve.
 9. The methodaccording to claim 1, wherein the automated cold cooling down procedurecomprises performing an adjustment step comprising simultaneously (i)adjusting and closing the compressor recycle valve; and (ii) furtheradjusting the first JT valve.
 10. The method according to claim 1,wherein the refrigerant recirculation circuit to recirculate the spentrefrigerant back to the cryogenic heat exchanger comprises a pluralityof compression stages with each compression stage comprising acompressor recycle valve and the adjustment step comprising adjustingand closing the plurality of compressor recycle valves.
 11. The methodaccording to claim 1, wherein downstream of the cooler and upstream ofthe first JT valve a liquid/vapor separator is provided in therefrigerant recirculation circuit, to receive a partly condensedrefrigerant stream and separate the partly-condensed refrigerant streaminto a liquid heavy refrigerant fraction (HMR) and a gaseous lightrefrigerant fraction (LMR) and to discharge the liquid heavy refrigerantfraction via a liquid outlet and the gaseous light refrigerant fractionvia a gas outlet, the method comprising the steps of the first JT valvecontrolling passage of the light refrigerant fraction and a second JTvalve controlling passage of the heavy refrigerant fraction.
 12. Themethod according to claim 11, wherein the initial opening step comprisesimposing the initial opening of the first JT valve and an initialopening of the second JT valve, wherein the initial opening step of thesecond JT valve according to the automated warm cooling down procedurediffers in size from an initial opening step of the second JT valveaccording to the automated cold cooling down procedure.
 13. The methodaccording to claim 12, wherein the automated cold cooling down procedurecomprises performing an adjustment step comprising simultaneously (i)adjusting and closing the compressor recycle valve; and (ii) furtheradjusting the first and second JT valves.